Long-term follow-up of Swedish children vaccinated with acellular pertussis vaccines at 3, 5, and 12 months of age indicates the need for a booster dose at 5 to 7 years of age.

OBJECTIVES The purpose of this work was to evaluate the long-term effectiveness of vaccination with acellular pertussis vaccines at 3, 5, and 12 months of age. METHODS Clinical follow-up of reported culture- and polymerase chain reaction-confirmed cases of pertussis was initiated during October 1997 in most of Sweden (except Gothenburg and environs). The study population included 90% of Swedish children born during 1996 or later (ie, who received diphtheria-tetanus-acellular pertussis vaccines at 3, 5, and 12 months of age) and children who had participated in a large pertussis vaccine trial in 1993-1996. Age-specific incidences were estimated using reported culture- or polymerase chain reaction-confirmed pertussis from October 1997 to September 2004 in areas covered by enhanced surveillance. In addition, annual overall and age-specific incidences of pertussis throughout Sweden before and after introduction of acellular pertussis vaccines were estimated. RESULTS The overall incidence of notified culture- and polymerase chain reaction-confirmed pertussis dropped from 113 to 150 per 100,000 during 1992-1995 to 11 to 16 per 100,000 during 2001-2004. In areas of enhanced surveillance, the incidence of pertussis was 31 per 100,000 person-years after 2 doses and 19 per 100,000 person-years after the third dose at 12 months of age. The age-specific incidence remained low for approximately 5 years after the third dose but increased in children aged 6 to 8 years, becoming 32 and 48 per 100,000 person-years, respectively. The highest incidence occurred among infants who were unvaccinated or had received only 1 dose of diphtheria-tetanus-acellular pertussis vaccine. CONCLUSIONS The increased incidence among 7- to 8-year-olds (ie, mainly acellular pertussis vaccine-vaccinated children) suggests waning of vaccine-induced protection from pertussis. Along with a concomitant increase in incidence among infants, most likely infected by older siblings, these data suggest a booster dose of acellular pertussis vaccine is warranted from 5 to 7 years of age.